1. Field of Invention
The invention relates to a lock-up control apparatus for an automatic transmission. More particularly, the invention relates to a lock-up control structure for smoothly performing shifting that requires phased switching of engagement and disengagement of the engagement elements between shift speeds.
2. Description of Related Art
An automatic transmission achieves multiple shift speeds by switching a power transfer route via a speed-changing element made up of a planetary gear set by engaging and disengaging the friction engagement elements and changing the gear ratio. It is desirable that the engagement and disengagement of the engagement elements during shifting be performed using the most simple oil pressure control possible while suppressing the occurrence of shifting shock. Therefore, in order to achieve a specific shift speed, generally one other engagement element is additionally engaged while a single or a plurality of engagement elements are in an engagement state, or one of the engagement elements that is engaged is disengaged as basic operations of engagement elements for shifting up and down. In addition, so-called change-over operations, during which an engagement element that is engaged is disengaged while another engagement element is engaged, are performed when necessary due to the gear train structure.
Recently, however, due to demands for improved drivability and for energy conservation by reductions in fuel consumption, there is a trend toward multi-speed automatic transmissions. Generally, these multi-speed automatic transmissions have an additional acceleration or deceleration speed due to an overdrive or underdrive gear in a speed change mechanism made up of a planetary gear set for multiple speeds. As disclosed in Japanese Patent Application Laid-Open No. 4-219553, however, as a different configuration, multiple speeds are achieved using two high and low systems as the input systems for a Ravigneaux type planetary gear set.
In a multi-speed automatic transmission such as described above, because the selection range for shift speeds that are suitable for driving states of a vehicle is expanding, not only a simple change-over operation of two engagement elements but also a complex change-over operation of four elements become necessary. An example of a case where this type of four-element change-over is required is a so called jumping shifting that shifts at once to a specific shift speed among multiple shift speeds. In particular, when performing multiple change-over that is represented by this type of four-element change-over, if each engagement element enters an engagement or disengagement state simultaneously, it becomes difficult to determine the behavior of each element of the speed change mechanism, and it becomes impossible to perform control practically. To overcome such problems, as shown in the time chart in FIG. 10, in the case of the shifting that involves the multiple change-over, it becomes necessary to control the servo oil pressure such that the engagement and disengagement of each engagement element (No. 1 through No. 4) occur in a specific order. As a result, shifting is performed in phases. That is, shifting is separated into a first shifting (A-C shifting) of a previous phase in which the disengagement of the first engagement element (No. 1) and the engagement of the third engagement element (No. 3) are switched, and a second shifting (C-B shifting) of a subsequent phase in which the disengagement of the second engagement element (No. 2) and the engagement of the fourth engagement element (No. 4) are switched.
Because the engagement and disengagement of each engagement element occur in a specific order, during the shifting that involves the multiple change-over, phased shock is likely to occur between the first shifting and the second shifting, and the shifting is likely to become slow. The occurrence of phased shock within the shifting and the slowing of shifting give an unpleasant sensation that is undesirable to the driver of the vehicle. To solve these problems, the oil pressure of the second engagement element that needs to be disengaged during the second shifting is lowered during the first shift control, so that the second shifting is started simultaneously with the completion of a change in the input rotation of the first shifting. Thus, the continuous multiple change-over shifting is achieved.
Another cause of the phased shock occurrence that cannot be solved with only the type of shifting control is the lock-up control of the hydraulic power transmission apparatus that transmits the power to the speed change mechanism.
Generally, in the case of a single-plate clutch, a lock-up control lowers a differential pressure applied to the clutch so that the clutch slips during the change in input rotation indicating the shifting from the previous shift speed to the subsequent shift speed. The lock-up control also starts the reengagement of the lock-up clutch by raising the differential pressure in phases simultaneously with the completion of the input rotation change that indicates the synchronization to the subsequent shift speed when the shift control is started in a lock-up on state. In a case where this type of control is performed during the multiple change-over shifting, as shown in the time chart in FIG. 11, when the shifting completion of the previous phase is detected through the input rotation change of the first shifting (A-C shifting), it is determined that the shifting is completed. Next, the control that increases the differential pressure of the lock-up clutch for the start of the reengagement is performed. Then, through the detection of the second shifting (C-B shifting) control start, the control that lowers the differential pressure of the lock-up clutch is performed again. Therefore, as shown by the dotted line in the time chart of FIG. 11, the engine rotation speed is instantaneously decreased due to the lock up during this period, and as a result the shift feel worsens. In order to solve these problems, an object of the invention is to provide a lock-up control apparatus for an automatic transmission that can prevent the occurrence of a phased shock by eliminating the lock-up on operation that occurs instantaneously during the shifting that involves the multiple change-over that starts during lock up.
The above object is achieved through a structure of a lock-up control apparatus for an automatic transmission according to a first aspect of the invention in which four engagement elements are required when shifting is performed from a first shift speed to a second shift speed, the first shift speed being achieved by engagement of a first engagement element and a second engagement element, the second shift speed being achieved by engagement of a third engagement element and a fourth engagement element. In an exemplary embodiment, the apparatus includes a lock-up control unit that performs a control for bringing a lock-up clutch into one of a disengagement state and a slip state at a start of shifting, and that prevents reengagement of the lock-up clutch at a completion of engagement of the third engagement element when shifting is performed from the first shift speed to the second shift speed during lock up.
According to a second aspect of the invention, the above object is achieved through a structure of a lock-up control apparatus for an automatic transmission which inputs power via a lock-up clutch, performs shifting by operations of a first through a fourth engagement elements when shifting is performed from the first shift speed to the second shift speed, and which continuously performs a shift control of a previous phase in which the first engagement element is disengaged and the third engagement element is engaged, and a shift control of a subsequent phase in which the second engagement element is disengaged and the fourth engagement element is engaged when shifting is performed from the first shift speed to the second shift speed. In another exemplary embodiment, the lock-up control apparatus includes a lock-up control unit that performs a control for preventing reengagement of a lock-up clutch at a completion of shifting of the previous phase, the lock-up clutch having been brought into one of a disengagement state and a slip state at a start of shifting when shifting is performed from the first shift speed to the second shift speed during lock up.